It is known for the purpose of measuring the NOx concentration in a measuring gas, for example the exhaust gas of an internal combustion engine, to use a sensor such as is described, for example, in the publication DE 199 07 947 A1. The mode of operation of the sensor is based on the Nernst principle. At temperatures above 350.degree. C., the solid electrolyte material of the sensor is simultaneously a very good oxygen ion conductor and a poor ion conductor with reference to other chemical elements.
Different oxygen concentrations on the two sides of the solid electrolyte lead to different electric potentials of the electrodes arranged on the respective sides. The potential difference then constitutes a measure of the difference in the oxygen concentration on the two sides of the solid electrolyte.
The quantity of remaining oxygen in the exhaust gas fluctuates strongly with a change in the air/fuel ratio, the lambda value. If the air/fuel mixture is in the so-called rich range (lambda value<1), in which the fuel is present in stoichiometric excess, the result is typically a Nernst voltage between the two electrodes of 800 to 1000 mV. For the so-called lean mixtures (lambda value>1), where the oxygen predominates over the air, the result is a Nernst voltage down to approximately 100 mV. Upon transition from rich to lean mixture, the Nernst voltage therefore changes discontinuously in the region about the stoichiometric lambda value 1 by 700 to 800 mV.
The measuring sensor in the abovenamed DE 199 07 947 A1 has two measuring cells in a body made from an oxygen-conducting solid electrolyte. Oxygen is pumped out of the first measuring cell, which is fed the measuring gas via a diffusion barrier, by means of a first pumping current, and a first oxygen concentration is set thereby.
The measuring gas diffuses from the first measuring cell into the second measuring cell via a diffusion barrier. The oxygen content is further lowered in the second measuring cell by means of a second pumping current, and an oxygen concentration is set. NOx is decomposed at a measuring electrode arranged in the second measuring cell, and the oxygen thereby formed is pumped off by means of a third pumping current. The third pumping current then constitutes a measure of the NOx concentration in the measuring gas.
In order to set the pumping current, the Nernst potential at the electrodes is tapped in the respective measuring cells, said potential being determined relative to the oxygen content of a reference gas to which a reference electrode is exposed.
Depending on the tendency to oxidation of the electrode material as a consequence of controller deficiencies or manufacturing fluctuations and/or material tolerances, it can happen that the electrode material oxidizes more or less strongly and changes its volume owing to the incorporation of oxygen. The corruption of the measurement signal associated therewith can have the effect that the emission values prescribed for motor vehicles can no longer be met using a sensor changed in this way.